RESUMO
Item responses to Digit Span and Letter-Number Sequencing were analyzed to develop a better-refined model of the two working memory tasks using the finite mixture (FM) modeling method. Models with ordinal latent traits were found to better account for the independent sources of the variability in the tasks than those with continuous traits, and the discretely distributed factors appeared to represent short-term storage (STS), general attention control (GAC), and the specific control mechanisms initiated by the interfering operations of mental sorting (MS) and backward ordering (BO). When related to the general ability factor (G) defined by the WISC-R verbal and performance scores and the total achievement score, the general working memory factors STS and GAC both seemed to share substantial variances with G, but the roles of specific factors MS and BO were less definitive. These WM factors accounted for the majority of the variability in G, with the multiple correlation between the factor mean scores of the WM factors and that of G above 0.80. Moreover, there seemed to be a discontinuity in the distribution of the ordinal GAC factor, as the two lowest subcategories of GAC were separated from the rest of the overall sample by the virtually empty third lowest subcategory, and the two outlying low subcategories contained the majority (80%) of the cases with mild mental retardation. The theoretical implications of these results were discussed.
RESUMO
The study subjected nine elementary cognitive task variables from the Cognitive Assessment Tasks (CAT) and three scholastic measures from the Metropolitan Achievement Test (MAT) to phenotypic and behavioral genetic structural equation modeling based on data for 277 pairs of same sex monozygotic (MZ) and dizygotic (DZ) twins from the Western Reserve Twin Project. Phenotypic and behavioral genetic covariation between certain elemental cognitive components and scholastic performance was examined to determine (a) whether these elemental cognitive components contribute substantially to the variance of scholastic performance; (b) whether such contributions vary across different domains of school knowledge or from specific domains to a general aptitude; (c) the behavioral genetic composition of the elemental cognitive components and the scholastic variables; and (d) how the association between the cognitive components and scholastic performance is genetically and environmentally mediated. The results of the study showed that as much as 30% of the phenotypic variance of scholastic performance was accounted for by the CAT general factor, which was presumably related to mental speed. A mainly genetic covariation was found between the mental speed component and scholastic performance, although each of the two variables was strongly influenced by both heritability and common family environment. The magnitude and etiology of the covariation were largely invariant whether mental speed was related to a common scholastic aptitude or to individual achievement measures covering different knowledge domains. Taken in conjunction with previous findings that mental speed has a substantial genetic correlation with psychometric g, and psychometric g has a mostly genetic covariation with scholastic achievement, the findings of the present study seems to point to a more global picture; namely, there is a causal sequence that starts from mental speed as the explanatory factor for both psychometric g and scholastic performance, and the etiology of the causal link is chiefly genetic.
